Mutations in the human lamin A/C gene cause dilated cardiomyopathy (DCM) (Brodsky et al., (2000) Circulation 101:473-6; Fatkin et al., (1999) N Engl J Med 341:1715-24; Taylor et al., (2003) Journal of the American College of Cardiology, 41:771-780), Emery-Dreifuss muscular dystrophy (Bonne et al., (1999) Nature Genet. 21:285-288), limb-girdle muscular dystrophy (Muchir et al., (2000) Hum Mol Genet 9:1453-9), partial lipodystrophy (Shackleton et al., (2000) Nat Genet 24:153-6), axonal neuropathy (De Sandre-Giovannoli et al., (2002) Am J Hum Genet 70:726-36) and mandibuloacral dysplasia (Novelli et al., (2002) Am J Hum Genet 71:426-31). Along with lamin B, lamins A and C are the major constituents of the nuclear lamina, a meshwork of protein filaments that underlies the nucleoplasmic face of the inner nuclear membrane. The nuclear lamina provides structural support for the nucleus (Newport et al., (1990) J Cell Biol 111:2247-59; Spann et al., (1997) J Cell Biol 136:1201-12), and plays a role in the regulation of gene transcription through direct and indirect interactions with transcription factors (Ozaki et al., (1994) Oncogene 9:2649-53; Markiewicz et al., (2002) Mol Biol Cell 13:4401-13; Spann et al., (2002) J Cell Biol 156:603-8), and by organizing intranuclear RNA splicing factor compartments (Kumaran et al., (2002) J Cell Biol 159:783-93). Lamins bind directly to DNA, and are involved in chromatin organization via direct interactions with histones and other chromatin binding proteins (Gotzmann & Foisner (1999) Crit Rev Eukaryot Gene Expr 9:257-65). Site-specific phosphorylation of lamin A results in the reversible disassembly of the lamina during mitosis (Haas & Jost (1993) Eur J Cell Biol 62:237-47), and lamin A is a target of endoproteolytic cleavage during apoptosis (Slee et al., (2001) J Biol Chem 276:7320-6).
Lamins A and C are differentially transcribed from the lamin A/C gene. Lamin A is expressed as a pre-protein (Gerace et al., (1984) J Cell Sci Suppl 1:137-60) that undergoes a sequential series of post-translational modifications (Sinensky et al., (1994) J Cell Sci 107:61-7) shared by the S. cerevisiae a-type mating pheromone (Marcus et al., (1990) Biochem Biophys Res Commun 172:1310-6), culminating in the endoproteolytic removal of the modified 15 amino acid residue C-terminal peptide. While proper processing of the prelamin A tail has been shown to affect the rate of mature lamin A incorporation into the nuclear lamina (Lutz et al., (1992) Proc Natl Acad Sci USA 89:3000-4; Izumi et al., (2000) Mol Biol Cell 11:4323-37), the physiological function of prelamin A processing has not been determined.
Lamins A and C are expressed in nearly all cell types concomitant with differentiation (Rober et al., (1989) Development 105:365-78). The reason why mutations in the lamin A/C gene result in tissue-specific abnormalities and the molecular mechanisms by which lamin A/C mutations exert their effects on these tissues has yet to be elucidated.
The initial cloning of lamin A/C indicated that the protein was processed. In the early 1990's, investigators began elucidating the processing pathway of prelamin A, and the localization and characteristics of enzymes involved in its processing (Lutz et al., (1992), supra; Dalton et al., Cancer Res. 55:3295-3304 (1995); Sinensky et al., J Cell Sci 107:2215-2218 (1994)). These researchers also investigated the biological function of the “pre” sequence by preventing its cleavage from the prelamin A protein, and inhibiting its processing in mononucleate cell lines. These studies demonstrated that the presence of the “pre” sequence prevented incorporation into the lamina, and also showed that mature lamin A lacking the pre-sequence could substitute for the native prelamin A without any biological consequences. In one of these studies, the authors comment that “nucleoplasmic localization of prelamin A or the peptide released during processing may have some regulatory significance” (Lutz et al., (1992), supra).
Investigators have reported the construction of a lamin A/C knock-out mouse that has cardiac and skeletal muscle phenotypes similar to those seen in patients with DCM and EDMD (Sullivan et al., (1999) J Cell Biol 147:913-20). Over the last two years, investigators have generated mice which lack the mouse homologues of the enzymes in the yeast Mat A processing pathways (Pendas et al., Nat Genet 31:94-99 (2002); Bergo et al., (2002) Proc Natl Acad Sci USA 99:13049-54). These animals appear to be phenocopies of the lamin A/C knock-out mice. These investigators have demonstrated that prelamin A is not properly processed in these animals.
Consequently, the published literature demonstrates that lamin A expression, and proper prelamin A processing are essential for normal post-natal cardiac and skeletal muscle biology in mice. The published data also shows that prelamin A must be processed to mature lamin A prior to incorporation into the nuclear lamina. However, there is no published data identifying the cellular function of prelamin A processing, or the mechanism by which mutations in the lamin A/C gene, or the deletion of the lamin A/C gene and enzymes that process prelamin A, lead to cardiac and skeletal muscle abnormalities. Such information would be invaluable for in the understanding of cardiac and skeletal muscle disease processes affected by lamin A/C disease mutations, as well as the ability to design therapies to prevent these and other cardiac and skeletal muscle diseases.